Within Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) transport networks, automatic protection switching (APS) enables working interfaces to be protected by backup interfaces. When a working interface fails, a backup interface assumes the traffic load of the working interface. In other words, APS provides the capability to detect a failure in an interface and to switch the traffic load of the failed interface to another interface.
Protection switching is typically implemented through the utilization of K1 and K2 bytes in a line overhead of a SONET or SDH signal. When a signal failure is detected or when signal degradation is detected, protection switching may be initiated. K1 and K2 bytes are used to effectively signal a line level protection switch.
With reference to FIGS. 1A and 1B, protection switching which uses K1 and K2 bytes in the line overhead of a signal will be described. FIG. 1A is a diagrammatic representation of a near end and a far end that are in communication over a plurality of working links and a protection link. That is, FIG. 1A depicts a 1:N protection scheme. It should be appreciated that N is generally an integer which has a value between one and fourteen, inclusive. Optical signals are typically sent from a source or a near end 102 to a destination or a far end 106 over working links 110a, 110b. A protection link 114 is generally not used until one of working links 110a, 110b fails. As shown, working links 110a, 110h and protection link 114 are bidirectional.
When working link 110a fails, as indicated in FIG. 1B, far end 106 detects the failure and sends a message using bits of a K1 byte to near end 102 over protection link 114. Generally, bits five through eight of a K1 byte are used to hold a switch action channel request. Hence, by sending a message in bits five through eight of the K1 byte, far end 106 requests that near end 102 switch from transmitting over working links 110a, 110b to transmitting over working link 110b and protection link 114. In response to the message sent by far end 106, near end may switch from transmitting on working links 110a, 110b to transmitting on working link 110b and protection link 114.
With reference to FIG. 2, the steps associated with implementing protection switching in a 1:N protection architecture will be described. A process 200 of implementing protection switching begins at step 204 in which a far end detects a failure on a working link between a near end and the far end. The working link has an associated protection link. Upon detecting a failure, the far end sends a message in a K1 byte of a frame to the near end in step 208. The K1 byte generally includes bits that indicate a switching priority and bits that indicate a requested switch action.
In step 212, the near end receives the message and switches traffic from the working link with the failure, i.e., the failed working link, to the protection link associated with the failed working link. Then, in step 216, the near end sends a message using a K1 byte and a K2 byte of a frame to the far end. Bits in the K2 byte are used to indicate a channel number for data traffic sent over the protection link, and bits in the byte are used to send a reverse request. The reverse request is typically used to initiate a bidirectional switch action.
The message sent by the near end is received by the far end, and in step 220, the far end switches to the protection link to receive traffic. After the far end switches to the protected link, the far end switches traffic from the failed working link to the protection link to transmit traffic in step 224. That is, the far end sets up to transmit packets, as well as to receive packets, using the protection link. Once the far end switches traffic to the protection link, the process of implementing protection switching is completed.
While the use of K1 and K2 bytes in SONET and SDH signals is generally effective for implementing APS, K1 and K2 bytes each only include one byte. The amount of information which may be transmitted using two bytes may be limiting in situations in which it would be desirable to transmit more information relating to APS. Further, K1 and K2 bytes are not transparent to a SONET or SDH cloud.
Therefore, what is desired is a method and an apparatus which allows information associated with a protection switch to be transmitted such that the information is not limited to a maximum of two bytes, and such that the information is transparent to a SONET or SDH cloud. That is, what is needed is a system which allows information typically associated with K1 and K2 bytes to be transmitted in bytes other than standard K1 and K2 bytes.